A corrugated board sheet is fabricated by the following process in which a liner (a bottom liner) is glued to a corrugated medium web with adhesive to form a single-face web, gluing the medium web of the single-face web to the other liner (a top liner) and then cutting the fabricated corrugated board into an appropriate length with a cut-off device. During the process, a web (each of a bottom liner, a top liner, a single-face web, and a corrugated board) is heated by a preheater exemplified by a bottom liner preheater, a single-face web preheater and a top liner preheater, and heated by a double facer, and is pasted by a single facer and a glue machine. At that time, an inappropriate level of heat or amount of glue causes defects in a resultant corrugated board sheet, e.g., width-direction upward or downward warp (hereinafter simply called width-direction warp) or inferior gluing. For example, an excessive moisture content of a bottom liner causes a convex surface toward a top liner when being dried; and an excessive moisture content of the top liner causes a convex surface toward the bottom liner when being dried.
Further, during the fabrication process, if a transfer-direction (hereinafter also called travel-direction) tension of a top liner or a single-face sheet is out of the appropriate range so that there is a significant difference in travel-direction tension between a top liner and a single-face web, the resultant corrugated board sheet has a defect such as travel-direction upward/downward warp (hereinafter simply called travel-direction warp) or inferior gluing.
Still further, when a travel-direction tension distribution (hereinafter simply called tension) of each web is varied in the web-width direction as compared with an appropriate distribution, a resultant corrugated board sheet has twist warp.
Generally, an optimum tension distribution (i.e., a tension distribution that causes no twist warp) is uniform over the web width direction. But, if fiber fabricating a web is inclined with respect to the travel direction of the web, the resulting corrugated board sheet has twist warp in spite of uniform a tension distribution in the web-width direction because the tension distribution is relatively varied in the width direction compared to the optimum tension distribution.
Conventionally, in order to correct warp of the above types, an operator visually checks warp status of a corrugated board sheet, appropriately selects one or more control factors that affect type of warp on the basis of experience or know-how and manually adjusts the individual control factors.
However, such an adjustment manner depending on experience or know-how may lead to inconsistency in quality of resultant corrugated board sheets depending on different operators's skill levels. Additionally, the same operator may repeat the adjustment over and over and may make errors in the adjustment operation leading to difficulties in obtaining constant product quality. Further, since there are a great number of control factors to be adjusted and adjustment variables of each control factor are determined considering current values, the adjustment operation is complex and time-consuming.
Engineers have been working on development of a technology that inhibits warp of the corrugated board sheets and thereby improves quality of resultant corrugated board sheets by matrix control that automatically adjusts each control factor such as wrap amount around each preheater, gap amounts in the single facer and the glue machine and pressure applied by the double facer, based on production state information such as a base-board composition, basis weight of the base board, width of corrugated board sheet, flute and the like so that width-direction warp is corrected. With such a system, a matrix prepared beforehand can deal with fabrication of corrugated board sheets even if a base board having a special composition or a specially processed base board is used for the fabrication.
During an exceptional production state in which a top liner and a bottom liner have the same base board composition and have different moisture contents or base boards of the same type are different in moisture content, even if each control factor is automatically adjusted in the above system, temperature of a web does not reach a predetermined appropriate temperature and, as a result, the resultant corrugated board sheets may have width-direction warp.
For this reason, warp caused under such an exceptional production state should be corrected by an operator visually checking warp status of the corrugated board sheet and dealing with the warp on the basis of experience and/or know-how.
In order to deal with travel-direction warp, Japanese Patent Application Laid-Open (Kokai) No. HEI 10-128881 discloses a technique in which appropriate tensions to be applied to a top liner and/or a bottom liner are calculated on the basis of a detection signal from a warp detection apparatus and tension adjusting apparatus adjusts the tensions thereof to those calculated.
However, since this technique simply controls one or more particular control factors, each of which has previously been selected as a tension adjusting apparatus, in accordance with warp status of a corrugated board sheet, adjustment of each control factor is constant irrespective of warp amount. Therefore, adjustment for only the above selected control factors takes a long time to correct warp of large extent and, in extreme cases, there is a possibility that travel-direction warp cannot be corrected.
A double facer presses a top liner, which is piled together with a single-face web, against hotplates to heat the single-face web and the top liner whereby the single-face web is heated through the top liner and is joined to the top liner.
A conventional double facer has hotplates, each of which is in a single form across the width of a web, so that the double facer cannot dissolve width-direction unevenness of the moisture contents (i.e., unevenness of temperatures) of a single-face web and/or a top liner transferred into the double facer. The moisture-content unevenness or the temperature unevenness may therefore tend to cause width-direction S-shape warp (hereinafter, simply called S-shape warp), that is, a sheet curling in a wave-shape in the width direction.
FIG. 86 schematically shows a front sectional view (seen from the web traveling direction) of a conventional preheater. The preheater 300 is in the form of a heating roll to heat web being wrapped around the roll during rotation thereof in synchronization with the travel of the web. The preheater 300 includes a cylindrical shell 301 into which vapor is supplied in order to heat web, axes 302a and 302b arranged at the both ends of the shell 301 and rotatably supported by bearings 303.
One axis 302a takes the form of a pipe through which vapor for heating a web is supplied into the inside of the shell 301. A drain pipe 304 is passed into the shell 301 through the axis 302a so that the vapor becomes condensation and is drained from the shell 301 through the drain pipe 304.
Similar to the above-described conventional hotplates included in a double facer, the conventional preheater is also in the form of a single form across the web-width direction (perpendicular to the web travel direction) and therefore cannot dissolve width-direction unevenness of water content (unevenness of temperature) of a base board (a top liner, a bottom liner, a medium web) transferred into a corrugated-board fabrication machine. The above moisture-content unevenness and temperature unevenness may sometimes cause a resultant corrugated board sheet to have width-direction S-shape warp.
Japanese Patent Application Laid-Open (Kokai) No. HEI 9-131814 discloses a technique to inhibit width-direction S-shape warp of a corrugated board sheet by dissolving width-direction unevenness of moisture content of the web.
In this technique, a plurality of press rolls, which press web in the process of being transferred, are arranged along the width direction of the web upstream or downstream of a preheater roll around which the web is wrapped. Each of these press rolls can be attached or detached the travel path of a web. Control for the position of each individual press roll varies tension acting on individual portions of the web which portions are arranged along the web width direction and also varies pressure applied to each of the portions of the web toward the preheater roll. It is thereby possible to vary a heat amount applied to each of the portions along the width direction and to dissolve water-content unevenness whereupon occurrence of S-shape warp is inhibited.
However, this technique largely affects tension of a web, e.g., variation in tension distribution in the width direction of the web, resulting in warp. There is a possibility that the S-shape warp cannot be satisfactorily inhibited and that warp of another type may be generated on a resultant corrugated board sheet.
As described above, an operator may correct warp of a corrugated board sheet by visually checking warp status (warp type and warp amount) of the corrugated board sheet, selecting one or more corresponding factors associated with the warp status from the control factors on the basis of experience and know-how, and manually adjusting the selected factors.
A visual operator check, however, cannot quantitatively grasp a warp amount of a corrugated board sheet, so it is difficult to accurately correct the warp of the corrugated board sheet as a result of such a visual check. Warp correction in this manner takes time until a corrugated board sheet of product quality can be obtained. Additionally, the operator has to continuously check status of corrugated board sheets that have been loaded.
In order to solve the above problem, an apparatus for quantitatively detecting warp amount of a corrugated board sheet has been developed. There has not been developed a technique to quantitatively detect an amount of travel-direction warp or twist warp.
A double-face web, which has been cut by a cut-off device to serve as a final product of a corrugated board sheet, is transferred from the cut-off device to a stacker by a conveyer and is stacked in the stacker.
In such a conventional corrugated-board fabrication machine, a counting roll that rotates following transfer of a double-face web is installed at a web transferring unit arranged upstream of the cut-off device. The number of corrugated board sheets is counted on the basis of amount of rotation of the counting roll.
The above counting roll, however, may be not able to obtain an exact number of fabricated corrugated board sheets because of slipping between the counting roll and transferred double-face web.
Further, corrugated board sheets having large warp or inferior gluing may sometimes be removed as being defective during transfer. If removal of defectives is carried out downstream of the counting roll, the number of corrugated board sheets obtained by the counting roll is consequently different from the number of corrugated board sheets serving as final products.